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Efficiency of an indirect water heater

hot_rod
hot_rod Member Posts: 23,275

Probably isn't anywhere near what many assume. AI seems to way off with their prediction of 99% :)

A coil in a tank of still water is not such a great heat transfer mechanism. Certainly not 90% boiler efficiency even if you have a massive or duals coil indirect for the boiler to feed.

I'd guess not much better than a boiler with a tankless coil, when there is no circulation.

The heat transfer number is not so easy to pin down with so many variables.

EK has it right using a separate flat plate, leveraging two moving, turbulent flows with a lot of surface area.

With close approach sizing of a plate HX you could generate 120 DHW with 125° SWT to the plate HX. So now you could leverage the mod con 90% efficiency. But still the crappy heat exchange inside the tank. A dhw recirc pump would change the transfer efficiency.

Condensing tankless WH would be a higher transfer efficiency moving turbulent flow again.

And of course the near 100% efficient electric tank water heater. Better yet a properly applied HPWH.

Nothing against indirect tanks, but the hype may be overlooking the thermodynamics.

Then there is the pie in the sky standby loss numbers. An indirect tank at 120F in a room at 80F skews those numbers. Put the indirect next to the combustion air grill and see how that less than 1° per hour works out. Ive been in boiler rooms with snow drifts below the combustion air grills.

Bob "hot rod" Rohr
trainer for Caleffi NA
Living the hydronic dream
mattmia2HomerJSmith
«1

Comments

  • mattmia2
    mattmia2 Member Posts: 10,854

    I think you eventually get good transfer but that is because you start out with poor transfer and the tank temp drops so the approach gets much wider and the transfer gets better. I've thought about either pre-firing the boiler on flow to the shower or adding a brazed plate hx before the tank at the inlet and in the return to the boiler.

  • Kaos
    Kaos Member Posts: 186
    edited December 2

    A plate HX (or coil inside tank) is pretty close to 100% efficient, any heat lost on one side is transferred to the other side. But this is not the reason why indirect efficiency is low.

    The issue with indirects is RWT. From monitoring my indirect, the return water tends to be above condensation temps, so at best an indirect is mid 80% efficient off a modcon.

    The other issue is losses during non-heat season. The modcon and all the piping has to be heated up to about 160F and all that heat is lost after each cycle as there is nowhere to purge it.

    This also assumes everything is plumbed and configured properly. How often do you see that in wild? Chances are the indirect is over pumped (thus higher RTW so lower efficiency) with high volume and uninsulated pipes. Add in some scale buildup on the heat exchanger and things get even worse.

    So I would say on a yearly efficiency basis a modcon+indirect is comparable to a power vented tank.

    It really makese no financial sense to install one from either BOM or operating cost. If you want efficient hot water of gas or propane, get a condensing tankless water hater.

  • EdTheHeaterMan
    EdTheHeaterMan Member Posts: 9,317
    edited December 3

    AI does have it wrong. The indirect efficiency is directly related to the efficiency of the heat source. Subtract a factor based on the efficiency of the transfer process including the surface area of the heat exchanging surfaces and any loss thru the insulated jacket to the ambient air. But to compare a tankless coil to an indirect is like comparing apples to pineapples. They both are apples but they are completely different fruits. And there are no pine cones involved.

    From experience I can tell you that an oil fired boiler with a tankless coil that is maintaining 160° in the off season will operate the oil burner for a few minutes every 4 hours as a result of the vent connection to the chimney. That same oil fired boiler will fire every 20 hours for a few minutes to recover the loss of temperature in the tank. That is assuming no one uses any hot water for that 20 hour period. (like when you are on vacation). I had a timer on my oil burner just to test this theory. It is not a theory anymore. it really happened with my Buderus indirect and my Buderus boiler with Riello burner over 30 years ago.

    That is why I recommend Indirect water tanks over tankless coils and tankless wall hung water heaters. You need to live like a European when you use a tankless water heater. Us Americans like to have as much hot water as we can get. 


    Edward Young Retired

    After you make that expensive repair and you still have the same problem, What will you check next?

    IronmanSuperTech
  • ethicalpaul
    ethicalpaul Member Posts: 6,572

    It really makese no financial sense to install one from either BOM or operating cost. If you want efficient hot water of gas or propane, get a condensing tankless water hater.

    I couldn't agree more except get a heat pump water heater

    NJ Steam Homeowner.
    Free NJ and remote steam advice: https://heatinghelp.com/find-a-contractor/detail/new-jersey-steam-help/
    See my sight glass boiler videos: https://bit.ly/3sZW1el

  • hot_rod
    hot_rod Member Posts: 23,275

    what us the factor to use?

    My comparison to a tankless coil was just about the heat transfer of a coil in still water, not the boiler operating confition. Although that is a big $$ number no doubt

    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
    EdTheHeaterMan
  • mattmia2
    mattmia2 Member Posts: 10,854

    unless this lets you get a stainless indirect tank that will last several times as long as the other options.

    EdTheHeaterMan
  • EBEBRATT-Ed
    EBEBRATT-Ed Member Posts: 16,400

    Tankless coils are ok for two people but in this day and age it doesn't make sense to me to keep a hot boiler 24/7.

    As far as indirect go I have an electric HWH. I have been on vacation for up to two weeks and come home to still have luke warm water after 2 week and mildly hot water if gone for 1 week and I shut it down when I leave. I assume indirects are insulated as good as a EWH.

    So I think indirects are a good choice.

    Tankless water heater seem nothing but trouble from liming up the HX due to high temp heat transfer and flow issues I know a lot of people who put them in and have ripped them out.

    HPWH are expensive to purchase and who can fix one if they bite the dust? A homeowner certainly isn't going to invest in the refrigerant tools needed for repairs and neither are the plumbers who install them. Many have given up on them as they just turn into an EWH. They are an expensive throw away appliance. Even if you get a rebate you wont get a second rebate

    MikeAmannSuperTechRevenantIronman
  • hot_rod
    hot_rod Member Posts: 23,275

    water heaters are big business, something like 4 million electric tanks are sold per year

    HPWH, around 190,000 in 2023

    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
    Revenant
  • ethicalpaul
    ethicalpaul Member Posts: 6,572

    nice graph @hot_rod

    I’d advise everyone to watch that acceleration curve in the years ahead

    NJ Steam Homeowner.
    Free NJ and remote steam advice: https://heatinghelp.com/find-a-contractor/detail/new-jersey-steam-help/
    See my sight glass boiler videos: https://bit.ly/3sZW1el

  • mattmia2
    mattmia2 Member Posts: 10,854

    Maybe htp can build one that will last long enough to pay for itself.

    LRCCBJ
  • jesmed1
    jesmed1 Member Posts: 668
    edited December 2

    @hot_rod Could you please comment on this efficiency comparison chart from Energy Kinetics? It's for different boiler types that also heat DHW. I don't doubt its correctness, but I wonder what assumptions are made for the conditions under which the non-EK boilers are running. Are these truly typical efficiency numbers for cast iron boilers with indirect or tankless coils?

    We have a natural gas fired water heater with a typical draft hood and standby losses up the flue, so we're probably 70% or less efficient there. I would love eventually to go to an Energy Kinetics System 2000 with an indirect tank that would (a) eliminate our existing 2 large cast iron boilers and (b) improve our hot water efficiency. I know Roger from EK has commented here before that the EK efficiency comes from (1) low thermal mass, (2) low water volume in boiler, and (3) post purge.

    Our combined system efficiency for DHW and cast iron boilers is probably around 70-75% now, and EK advertises 85% combined system efficiency for the System 2000 with DHW, so in theory we could probably improve our efficiency 10-15% by going to a System 2000 at 85% system efficiency.

  • hot_rod
    hot_rod Member Posts: 23,275

    I don't know who compiled that chart. I usually look for independant lab test results.

    Be handy to see mod con with indirect.

    There are so many variables however, for comparisons like this.

    Cycle efficiency tells more of a story. I did a post on calculating that recently.

    Cycle Efficiency = total heat output over a period of time ÷ energy content of fuel consumed

    Cycle efficiency will always be less than steady state efficiency, calculate it from the run fraction

    Run Fraction burner on time ÷ total elapsed time

    Mild temperature days with fixed output boilers is where it fall apart.

    Using load info RF= heat required ÷ steady state output

    A 30,000 load on a 125,000 boiler = .24 or 24%

    Using this Brookhaven developed graph, (used with permission) that RF % puts you in the 70% range.

    Therein lies the problem with zoned boilers in mild conditions, or less than design load. Which may be 80% of the heating season.

    Don't dismiss the importance of modulation, even running out of condensing mode.

    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
    jesmed1LRCCBJ
  • EBEBRATT-Ed
    EBEBRATT-Ed Member Posts: 16,400

    HPWH is 2% of the market. Not popular and not perfected yet. When they are done experimenting and start to sell more the price may come down

    MikeAmann
  • GroundUp
    GroundUp Member Posts: 2,091

    I've been a long time proponent of indirects over anything else for fully occupied homes, but began questioning this efficiency not so long ago after paying attention to the delta across the coil in multiple different brands and sizes of indirect. My personal home, I use an outdoor wood boiler for space heating and have a plate heat exchanger piped into the cold inlet of my tank which I feel would be a great option in lieu of an indirect when using a mod/con, possibly with a parallel circuit to the space heating zones(s) so that any DHW draw during a space heating cycle would bring in preheated water to the tank. I've also been kicking around the idea of adding a small bronze or composite circ to mix the indirect tank (from the relief port to the drain port with tees) to keep a flow of water across the indirect's internal coil as well as heating it in a parallel fashion via plate exchanger to maximize heat transfer and efficiency. Probably a negligible increase on both accounts, but it's something…

  • hot_rod
    hot_rod Member Posts: 23,275

    On one hand you will increase the heat transfer of the indirect considerably by moving both flows

    However the trade off is you lose the tanks stratification, which can add 7, 10 or more gallons of drawdown

    With a wood fired boiler you could run the indirect to 140, 160f or more mix down to 120

    I guess it depends on your dhw needs and usage. If you have a large soaking tub or multiple loads in a row, that extra drawdown can be nice to have

    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • GroundUp
    GroundUp Member Posts: 2,091

    I was referring to a mod/con system, not my own.

  • hot_rod
    hot_rod Member Posts: 23,275

    Back to the topic

    Standby loss, my HeatFlo, it was at 116F at 7:00 pm. At 7:00 am this morning it is at 106F

    Sitting in a 67F shop. So the “less than a degree per hour” holds in this application

    It is only only connected to a drainback, water drains out, so no potential to thermo siphon heat away

    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
    HeatingHelp.com
  • GroundUp
    GroundUp Member Posts: 2,091

    How the heck did this turn into sexist comments??? Hopefully somebody is no longer a member here if that's how they behave on a public forum.

  • HeatingHelp.com
    HeatingHelp.com Member, Moderator, Administrator Posts: 65
    edited December 4

    They are no longer a member here. Let's get back on topic. Thanks.

    Forum Moderator

  • Doug_7
    Doug_7 Member Posts: 257

    The actual efficiency of a system, can be determined by measuring the actual heat losses of its components.

    Attached is a chart showing the efficiency of an actual 80 gallon Direct Fired DHW Tank with the total heat losses broken down into three major categories. In this system, only 47% of the Total Heat goes into heating DHW for use. The rest of the Heat goes into Standby Heat Losses, Recirculation Heat Losses and Flue Heat Losses.

    Timers were used to collect the data on energy use for Standby and Recirculation Heat Losses during a fixed period at night when there was no actual DHW use.

    This is the actual data on an old Direct DHW Tank system, just before it was converted to a stainless-steel Smart Indirect Tank fired by a ModCon boiler with much higher thermal -efficiency.

    Indirects also have much lower Standby Heat losses.

    jesmed1
  • hot_rod
    hot_rod Member Posts: 23,275

    when it was converted to indirect was the piping, including the recirc line done then also?

    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • jesmed1
    jesmed1 Member Posts: 668

    You say the measurements were taken during night when no DHW was being used. Does that mean your system had hot water recirculation for instant hot water at each tap? For systems like mine with no recirculation, does that mean our recirculation loss would be zero? I assume there's some heat loss anyway in hot water flowing to each tap even without recirculation, but maybe that's hard to quantify.

  • Doug_7
    Doug_7 Member Posts: 257

    Hi jesmed1 -

    The measurements were taken at night during a standard 5 hour period when no DHW was being used. Some nights the measurements were taken with the DHW Recirculation ON which gave Standby + Recirculation Losses. Other nights measurements were taken with the DHW Recirculation OFF which gave only the Standby Losses. The difference between these readings is the Recirculation heat losses.

    A Red Lion timer measured the firing minutes during a standard 5 hour period.

  • Doug_7
    Doug_7 Member Posts: 257

    Hi jesmed1 -

    Yes - Your heat loss without recirculation would be zero. The supply piping cools down and the heat losses drop to zero.

    In large systems with recirculation, the heat losses due to recirculation can be significant, because it keeps both the Supply and Recirculation piping hot 24 / 7.

  • jesmed1
    jesmed1 Member Posts: 668

    OK thanks, very interesting. Our 75,000 BTU/hr natural gas water heater is listed as 80% efficient with about 2.5%/hr standby loss, or a bit over 1000 BTU/hr standby loss (per the tag).

    So I guess if our heater burned for 1 hr, using 75,000 input BTUs, and then idled for 9 hours, we would expect a 25% standby loss (2.5%/hr x 10 hrs total).

    But we have our water temp dialled down low, so I expect our standby loss would be less than listed, so say 750 BTU/hr. Then over 1 year, we would lose 750 BTU/hr x 24 hrs/day x 365 days/yr = 6,570,000 BTU.

    Then 6570000 BTU/100,000 BTU/therm = 66 therms.

    Here in Boston a therm costs us $1.50. So that's 66 therms x $1.50/therm = $99/year in standby losses.

  • Doug_7
    Doug_7 Member Posts: 257

    hot_rod - when it was converted to indirect was the piping, including the recirc line done then also?

    No - The original system was an 80 gallon Direct Fired Tank with Recirculation - that is what you are looking at.

    The 80 gallon Direct Fired Tank was replaced by a 120 gallon Smart Indirect which greatly reduced Flue Losses and Standby Losses.

    The Recirculation system was converted from flow-control to temperature-control using CircuitSolvers from ThermOmegaTech primarily to prevent pin-hole water leaks. The recirculation pump was reduced from 10 GPM to 2 GPM at this time. Better control reduced recirculation heat losses.

  • ChrisJ
    ChrisJ Member Posts: 16,289

    @Revenant

    Ethanol is primarily used as an octane booster rather than MTBE and before that lead, so it's not hard to pin down.


    That said, wow.

    Single pipe 392sqft system with an EG-40 rated for 325sqft and it's silent and balanced at all times.

    GGrossethicalpaulKC_Jones
  • hot_rod
    hot_rod Member Posts: 23,275

    It's always about the ∆ .

    My tank at 116 in a 67° degree space has a lower loss than a tank at 140 in a 55° basement, for example.

    On a fired tank the loss up the flue depends on the draft I suppose., In addition to loss through the tank insulation.

    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
    GGross
  • ChrisJ
    ChrisJ Member Posts: 16,289

    It's always funny to see those huge blankets wrapped around atmospheric tank heaters. On an electric tank, or maybe even a power vent it could help, but on one just blowing heat up the chimney……

    Single pipe 392sqft system with an EG-40 rated for 325sqft and it's silent and balanced at all times.

  • Doug_7
    Doug_7 Member Posts: 257

    hot_rod - On a fired tank the loss up the flue depends on the draft I suppose., In addition to loss through the tank insulation.

    On a direct fired tank, unless you have a flue damper, which my example does not have, most of the standby heat loss is up the flue, rather than through the tank insulation. When the tank is not firing, heat losses are still drafting up the flue at a great rate.

  • jesmed1
    jesmed1 Member Posts: 668

    "When the tank is not firing, heat losses are still drafting up the flue at a great rate."

    I've observed this on our water heater, for sure. We have an atmospheric water heater plus two cast iron boilers vented up the flue, and none of them have vent dampers. So the basement is chilly the entire heating season, and I can put a flame up to the water heater flue and see our semi-warm basement air getting sucked right out of the building. And when that happens, heated air from the first floor gets sucked down into the basement.

    So now I'm more interested in stopping the air exfiltration up the chimney than I am in getting better efficiency by installing condensing boilers. I think for us the optimal setup would be two boilers with automatic vent dampers, and one of the boilers running an indirect water heater. That way there are no more leak paths for warm air exfiltration, and I wouldn't be surprised if we cut our fuel usage by 10% or more.

  • Roger
    Roger Member Posts: 371

    I'll chime in here - much of the questions can be answered with the BNL study as it published "summer domestic hot water efficiency" results for indirect tank systems with cast iron (38.3% with 84 AFUE to 68.3% with 89 AFUE) and condensing boilers (58.7% with 95 AFUE), tankless coil boilers (31.0% with 84 AFUE to 40.6% with 83.9 AFUE), direct fired water heaters used for heating and hot water (57.1% with EF 68 and 57.5% with EF 59), and of course a non-condensing low mass boiler with thermal purge and external plate heat exchanger (74.9% with 87.5 AFUE). There was also an imported oil combi boiler tested (47.9% with 82.8 AFUE); this should not be compared to low mass condensing gas combis.

    A couple of comments:

    1. Low mass with thermal purge and a plate heat exchanger has high efficiency when the burner is running, and virtually eliminates standby losses with a very effective thermal purge. The low mass with thermal purge to a storage tank is even more efficient for condensing boilers as it pulls cold makeup water from the bottom of the tank and can condense through nearly the entire hot water tank heating cycle. With the oversized plate heat exchangers Energy Kinetics uses, the boiler can maintain return temperatures as low as 80°F, but typically they are set at 110°F or lower for exceptional condensing and performance. As AFUE is tested near 120°F return temperatures, that means the boiler efficiency in this mode is even higher than shown with AFUE. Thermal purge means that very little energy is left wasted at the end of the hot water cycle (it can purge against a reservoir of cold water at the bottom of the tank to heat the water from energy left in the boiler after the burner turns off).
    2. Indirect tanks have coils that heat up as the tank water heats, so the return water temperature to the boiler increases. This really hurts the efficiency of condensing boilers, and moderately hurts the efficiency of non-condensing boilers. Higher mass means more heat wasted in the boiler when the burner turns off; heavy insulation can help.
    3. The best efficiency of a storage tank (no open flue) and accounting for standby losses is around 95%. This is probably well represented by an electric tank UEF; these are between 0.92 and 0.95, so 5% to 8% standby losses.
    4. Tankless coil boilers are very inefficient, and have poor hot water delivery, we can do better and always should.
    5. Chimney and vent losses occur in the off cycle because boilers without thermal purge finish hot. With thermal purge, these losses are also nearly eliminated. The hot water purge cycle only needs about 5 minutes to capture the energy left in the boiler, then there's no more energy to waste up the chimney; no need for draft dampers, etc.

    Best,

    Roger

    President
    Energy Kinetics, Inc.
    jesmed1
  • Doug_7
    Doug_7 Member Posts: 257

    jesmed1 - and I wouldn't be surprised if we cut our fuel usage by 10% or more.

    I agree on the "by 10% or more."

    One of the most overlooked advantages of ModCon boilers is that they have their own combustion air-intakes, and the open combustion air intake to the boiler room is no longer required. So no more drafting heat losses up the old flues 24 / 7.

    Ours is a small-commercial operation and with the installation of ModCon boilers 15 years ago, we only have one natural-draft boiler left - for driveway melting - and it has its own combustion-air damper which has to open first for that boiler to fire.. Our boiler room is warm, not hot, and draft free most of the winter.

    You have to be careful to meet combustion air requirements for all boilers.

    .

  • jesmed1
    jesmed1 Member Posts: 668

    Thanks, Roger. I always learn from your informative posts. Someday I hope to be fortunate enough to own an Energy Kinetics boiler!

    Roger
  • jesmed1
    jesmed1 Member Posts: 668

    @Doug_7 Thanks for that info. You're giving me more confidence that eliminating the atmospheric drafts is our next logical step in energy efficiency. We can do that with an automatic vent damper on each boiler, and then plumbing in an indirect water heater.

  • CoachBoilermaker
    CoachBoilermaker Member Posts: 340

    Can someone summarize big ideas for a dummy? I have tankless coil oil boiler.

    If I replace my boiler, I should

    1) get an indirect water tank heated by the oil boiler or

    2) Electric hot water tank?

    Both of these are better than tankless coil ?

  • SuperTech
    SuperTech Member Posts: 2,409

    Depending on your energy costs, a tankless coil boiler is usually the worst option. Electric water heater vs indirect depends on the fuel costs but I would rather have an indirect water heater personally.

  • hot_rod
    hot_rod Member Posts: 23,275

    It's a toss up between electric resistance and corn :)

    Go to this site and plug in your actual energy costs. Boiler at 85%, electric resistance at 100%. For heat pump water heater use the geothermal line with expected COP

    From a sheer operating cost the HPWH probably wins.

    https://coalpail.com/fuel-comparison-calculator-home-heating

    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • Doug_7
    Doug_7 Member Posts: 257

    Hi jesmed1 -

    Look up HOYME Combustion Air Damper or local equivalent.

    jesmed1